From The Life of Birds, David Attenborough, 1998, BBC Books, London:

The most elegant nests of all are those that are woven. To make these, the beak serves as a shuttle, threading a thin ribbon stripped from a leaf, or a tougher fibre made from a leaf's midrib, alternately over and under other strips. And this is not the only skill that is required. A bird must be able to assess how taut each strip should be pulled, and envisage the final shape the nest will take so that it can judge when the curve of the walls must be brought in or bellied outwards. In most bird species it is the female who tackles the bulk of the construction work and that is the case among the South American birds who have taken up the craft of weaving - orioles, oropendolas and caciques.

A female oropendola begins by wrapping the end of a long strip, torn from a leaf, around the dangling twig of a tree. She may hold it against the twig with one foot while manipulating the free end with her beak. She secures it by pushing the short end through the wrapping and so tying a half-hitch. Then she takes the long end and curves it round and back to the first knot. She may do this with several strips in the same place. The resulting hoop will form the entrance to the finished nest. From this she weaves the walls, hanging downwards and working from the inside, using her beak to thrust one fibre under another, and then delicately grasping the free end and pulling it tight, so creating an interlaced fabric of great neatness and uniformity. As she works, so she creates a tube that extends downwards from the entrance. When she flies away to collect another fibre, she does not take off from the open bottom of the tube, but clambers up inside it and departs from what will become the proper entrance. As the tube lengthens, she widens it so that it becomes club-shaped. Only when it is six feet long will she bring the curve inwards and seal off the bottom.

In Africa, the pre-eminent weavers are members of the sparrow family. Here - unusually -- it is the male who makes the nest. The many species include the masked weavers, greater and lesser, that use their nest-weaving skills as a means of attract-ing a female; the grosbeak weaver that builds an outstandingly neat construction, using only thin fibres shredded from one particular grass and tucking in the edges of the side-entrance to form a selvedge; and another, the sociable weaver, that is more of a stacker than a weaver. It builds not a pouch for a single clutch of eggs, that is used for only a few weeks in the breeding season, but a giant apartment block that is occupied the year round by as many as a hundred families. Sociable weavers live in the deserts of south-western Africa. Their homes are massive stacks of coarse dry grass that may be a century or so old and weigh several tons. Some are so bulky and heavy that the branches of the tree in which they are built break under the weight and bring the whole construction crashing to the ground. The birds live in tunnels that open on the underside of the stack. Some of these lead up into nesting chambers; others are cul-de-sacs in which birds may roost. The upper surface is thatched with a layer of rather coarser stems. All the birds collaborate to build and repair this feature from which all benefit.

Such a construction could only exist in a desert where rainfall is very low. If it were soaked by heavy rains, the whole stack would rot and disintegrate. Here, however, it protects its builders from the extremes of the desert conditions. During the day, its great thickness shields those within from the heat of thc ferocious sun; and at night, perhaps even more valuably, the nest chambers retain much of their daytime warmth, while outside in the desert the temperature drops to several de-grees below freezing.

No matter how beautifully made, ingeniously designed or artfully suspended from the branches of tall trees, few nests are beyond the reach of all predators. The oropendolas acquire insect guardians by regularly building their nests beside those of wasps that hang from trees like papery inverted mushrooms. Such wasps have very powerful stings, certainly venomous and painful enough to deter any unprotected human being from disturbing them. Initially, when the oropendolas start building, the wasps may attack them, but within a day or so, the insects become accustomed to their new neighbors and although the birds fly busily to and fro dur-ing the nesting season and often cause quite a commotion with their squabbles, the wasps ignore them. It is only when an intruder such as an opossum or a snake makes its way down the branch towards a suspended nest, or a raiding toucan flies in that the wasps go into the attack. Only a very brave and hungry thief would be undeterred by them. Just how the insects distinguish between the birds in the col-ony and an intruding bird is not certain, but it is the case that oropendolas and other birds that form these associations have a strong musty body smell which others, including even closely related species, lack. The insects may themselves derive some benefit from the relationship. Caracaras, a species of hawk, regularly attack wasp colonies as do tamandua anteaters and when either appear near an oropen-dola colony, the birds will vigorously attack them and so protect their protectors.